A circle has a radius of $4$. An arc in this circle has a central angle of $\pi$ radians. What is the length of the arc? ${8\pi}$ ${\pi}$ $\color{#DF0030}{4\pi}$ ${4}$
Explanation: First, calculate the circumference of the circle. $c = 2\pi r = 2\pi (4) = 8\pi$ The ratio between the arc's central angle $\theta$ and $2 \pi$ radians is equal to the the ratio between the arc length $s$ and the circle's circumference $c$ $\dfrac{\theta}{2 \pi} = \dfrac{s}{c}$ $\pi \div 2 \pi = \dfrac{s}{8\pi}$ $\dfrac{1}{2} = \dfrac{s}{8\pi}$ $\dfrac{1}{2} \times 8\pi = s$ $4\pi = s$